In particle beam therapy a tumor in a patient is irradiated with protons or ions in a pulsed manner. This pulsed proton or ion radiation is supplied in this context by a particle accelerator, for example a synchrotron. Radiation pulse sequences, having an infinite number of radiation pulses, alternate here with beam delays. The radiation parameters here are set in such a manner that the greatest degree of destruction possible of the tumor tissue is achieved with the smallest possible degree of damage to the surrounding healthy body tissue. They are determined before the radiation therapy takes place. In order to be able to control the radiation dose applied at the application site or, respectively, at the site of the tumor, the applied radiation dose is determined in a locally resolved manner using a positron emission tomograph. This makes use of the fact that positron emitters are formed at the application site by nuclear conversions during the radiation process. Every positron emitter releases a positron as it decays, said positron together with an electron annihilating with an energy of 511 keV respectively in two x-ray quanta. These x-ray quanta are measured by the positron emission tomograph. An evaluation algorithm is used to determine the respective site of origin of the x-ray quanta from this location information. The radiation dose applied at the application site is calculated in a locally resolved manner by temporally integrating decay events in the radiation period. It is thus possible to verify, in a locally resolved manner, the extent to which the radiation dose actually applied differs from the planned radiation dose. It is thus possible to adapt the radiation parameters for further radiation processes.
The positron emission tomograph has a processing unit, in which two tests are carried out, to establish whether two detected x-ray quanta can be traced back to a positron decay. On the one hand it is verified whether the two x-ray quanta lie in a narrow time window, what is known as a coincidence window, of around 2-10 ns duration. In a second test it is verified whether the x-ray quanta respectively have an energy of 511 keV. Since the accuracy of the energy measurement is limited, x-ray quanta, which originate from am energy window of around 350 keV to 650 keV, are assigned to a positron decay.
A problem arises during the evaluation of the measured x-ray quanta. During the interaction of the particle beam at the application site the energy input due to the protons or ions causes further numerous nuclear reactions to be induced, which similarly result in the emission of x-ray quanta. If two such x-ray quanta lie in a common coincidence window, positron decays are simulated. Therefore the positron emission tomograph is not generally used for measurement purposes during the interaction of the pulsed particle beam with the body tissue. Measuring only takes place in the beam delays between the radiation pulse sequences. Since these beam delays only make up around 30 to 70% of the total beam exposure time, compared with the radiation pulse sequences, also referred to as beam extraction phases, and since positron decays are not taken into account during the beam extraction phases, measuring using positron emission tomographs is subject to relatively major error. This error is perpetuated in the calculation of the radiation dose applied in a site-dependent manner.
Generation of x-ray quanta, which are not attributable to a positron decay, takes place almost exclusively during the interaction of a radiation pulse with the body tissue. The radioactive isotopes occurring in the body tissue in addition to the positron beams are so short-lived that their decay takes place during or immediately after such a radiation pulse. A precise temporal identification of the time intervals, in which the radiation pulses interact with the tumor tissue, would also make it possible to evaluate the coincidence events, which occur in the time intervals between radiation pulses. This would allow a decisive improvement in the evaluation statistics.
The radiation pulse sequence is known, since the particle accelerator receives a corresponding control signal, generated by a control unit, but the protons or ions first cover a free flight distance from the site of origin or reference site in the particle accelerator to the application site, in other words to the tumor to be irradiated, followed by a path inside the body. Since the decay of the radioactive isotopes also takes a specific time, it is not clear from the x-ray quanta measured as a function of time, at which time exactly the interaction of a radiation pulse with the body tissue takes place.
In P. Crespo et. al., “Suppression of Random Coincidences During In-Beam PET Measurements at Ion Beam Radiotherapy Facilities”, IEEE Transactions on Nuclear Science, Vol. 52, 2005, page 980 ff two methods are proposed for determining the times of the radiation pulses and therefore the times of the delays between the radiation pulses.
With the first method an additional detector in the beam path is used to measure when a radiation pulse occurs. Since the additional detector is disposed close to the application site, there is only a small time offset between the detector site and the application site. Very precise synchronization is required here between the measurement signals measured by the positron emission tomograph and the measurement signals of the additional detector. This requires a complex electronic unit.
With the second measuring method the control signals, with which the particle accelerator is triggered by way of the control unit, are evaluated. The radiation pulse sequence is therefore known. An electrical component, what is known as a phase trigger, is used to determine the delay between the reference site and the application site. This method also requires very complex and precise synchronization between the control signal of the particle accelerator and the positron emission tomograph.
With both methods described those time intervals are determined, in which an interaction takes place with the particle beam at the application site. Those coincidence events, the recording time of which lies in such an interval, are then rejected, in other words not used for an evaluation. Determination of the time intervals is however very complex in both instances.